Negative SEO exists. I can testify to that. On 4th October 2013 we were hit badly. Lost nearly all Google traffic. I've always been white hat, do never paid much attention to my incoming links. I have found about 650 spammy links so far. They consisted mostly of forum postings and blog commenting, basically stuffing my keyword as the anchor text, with a combination of offensive words f*** and tw*t.

In my case, I do not believe it was a competitor. Instead, an unreputable SEO firm selling their services to recover from Penguin. I received about 3 emails over the course of 6 months about such services. Prior to my penalty, I'd never received these emails.

It's scandalous that Google has made me (and many others) almost a year, with still no sign of a refresh. It's changed my life for the worse considerably.

I think google will be using metrics such as bounce rate sooner or later.

The bounce rate will be higher if the page speed is slow (i've experienced it myself). So even if page speed is not given any weight in the google algorithm, it could still potentially in turn affect your ranking due the this reason in the future.